This invention relates to prismatic lighting panels, and in particular to a light-weight plastic lighting panel used for the distribution of light from a light source. Prismatic lighting panels are widely used in overhead fluorescent lighting fixtures, and may be used with other light sources. Their primary purpose is to reduce direct glare by controlling the angle at which light emerges from the panel.
The theory of prismatic lighting panels is well known, and is discussed, for example, in McPhail, U.S. Pat. No. 2,474,317. Such panels include a planar upper face and a lower face covered with prismatic elements. Light rays entering the top of the panel are either refracted downward through the lower surface of the panel at useful angles to the vertical (i.e. the normal of the panel), or are reflected internally by the prismatic elements upward through the upper surface of the panel. If the prismatic elements have straight sides which make the proper angle with the normal of the panel, virtualy all of the light which would otherwise emerge at high angles relative to the normal of the panel is internally reflected by the prisms and high angle "direct" glare is thereby greatly reduced or eliminated.
A particularly popular prismatic lighting panel has, on its lower surface, female conical prisms, the apexes of which are aligned along 45.degree. diagonals to the edges of the panel and spaced three-sixteenths of an inch (0.50 .+-. 0.05 centimeters) on centers. The intersections of the cones thus form a structure of square cells, all of whose sides lie along lattice lines running at angles of 45.degree. to the edges of the panel. An example of such a lighting panel is one sold by K-S-H, Inc., under the trademark KSH-12. For convenience, a panel having this pattern will be referred to herein as "the usual" panel.
In recent years there has been an increasing demand for inexpensive prismatic lighting panels. Because the plastic material of which the panels are made represents the major cost of prismatic lighting panels, the usual prismatic panel has been made ever thinner, until presently it has reached the limit permitted by its geometry. Because the apex angle of the female prisms is critical to the optical performance of the panel, the height of the prisms is a function of the size of the individual cells of the prismatic pattern, that is, the spacings between apexes of the cones. Although it is theoretically possible to reduce the size of the cells below the three-sixteenths inch side of the usual panel, both aesthetic considerations and manufacturing constraints have made any substantial decrease in cell size impractical. Other techniques for reducing the amount of material in a panel have been attempted, such as increasing the apex angle of the female cones or "hogging out" the prisms so that their surfaces are concave rather than linear in cross section, but these techniques yield only limited savings of material and tend to degrade the optical performance of the panel. Using presently known techniques, commercially available embodiments of the usual panel have been reduced to an overall thickness of about 0.093 inch (0.236 centimeters), and have been made as light as 5.4 ounces per square foot (1650 grams per square meter).
One of the objects of this invention is to provide a prismatic lighting panel which has optical characteristics at least comparable with those of presently known light-weight embodiments of the usual panel, and which may be made substantially lighter in weight than such panels.
Another object is to provide such a panel which resembles the usual panel in use, and which is as aesthetically pleasing as the usual panel.
Other objects will become apparent to those skilled in the art in light of the following description and accompanying drawings.
In accordance with this invention, generally stated, a prismatic lighting panel is provided having a planar upper face and prismatic lower face, the lower face defining a plurality of intersecting recessed cones having apex angles of about 116.degree. (.+-.10.degree.), the apexes of the cones being spaced from each other such that the intersections of adjacent cones form sides of equilateral triangular cells arranged along lattice lines extending generally at 60.degree. to each other. The sides are preferably about three-sixteenths inch (0.50 .+-. .05 cm.) long. The intersections of the lattice lines are the lowermost points of the lower face, and in the preferred embodiment the vertical distance from the apexes to a plane defined by the lowermost point is about 0.065 inch (0.17 cm.). The overall thickness of the panel may be anything greater than the height of the prisms, but is preferably from about 0.07 inch to about 0.10 inch. The panel may be formed of any transparent material, preferably a thermoplastic material such as an acrylic or light-stabilized polystyrene.
Because of the triangular configuration of the cells, the height of the prisms (i.e. the vertical distance from the apexes of the recessed cones to the plane defined by the lowermost points) is considerably less than the height of square-celled prisms having sides of the same length. Therefore, the panel of the invention may be considerably thinner and lighter than the usual panel, although both have prisms which are three-sixteenths inch on a side. For example, with an apex angle of 116.degree., the prisms of the present panel have a height of 0.067 inch (0.17 cm.), whereas those of the usual panel have a height of 0.084 inch (0.21 cm.). Therefore, the preferred embodient of acrylic panel of the present invention having a 0.1875 inch (0.48 cm.) cell side and a total thickness of 0.085 inch (0.22 cm.) has a "base" thickness, between the apexes of the recessed cones and the upper surface of the panel, of 0.018 inch (0.05 cm.) and weighs about 4.8 ounces per square foot ( 1.46 kilograms per square meter). The usual panel, with square cells of the same length side, requires a total thickness of 0.102 inch (0.26 cm.) to provide the same base thickness and would weigh about 6.2 ounces per square foot (1.89 kilograms per square meter), if the previously mentioned weight-saving techniques were not used.
The difference in weight between the usual panel and the panel of the present invention not only reflects a difference in the overall thickness of the panel required for a particular base thickness, but also indicates that the prisms of the present panel require less material for a particular prism height than do the prisms of the usual panel. Therefore, the weight advantages of the panel of this invention may be enjoyed over a considerable range of prism sizes and panel thicknesses. The use of three-sixteenths inch cell sides is preferred because it gives the panel an appearance similar to that of the usual panel. The approximately 116.degree. apex angle is preferred because it provides superior optical characteristics. If the prism cell size is increased, the overall thickness and weight of the panel are also increased, but they may still be less than the corresponding values for the usual panel. For example, if the cell side is increased to 0.20 inch (0.05 cm.), the prism height is increased to 0.072 inch (0.18 cm.), the overall panel thickness may be 0.092 inch (0.23 cm.), and the overall weight will be about 5.1 ounces per square foot (1.47 kilograms per square meter). This is still considerably lighter than the lightest available commercial panel of the "usual" type. If the cell side is increased to 0.25 inch (0.64 cm.), the prism height is 0.090 inch (0.23 cm.); with a 0.015 inch (0.04 cm.) base thickness, the overall panel thickness will be 0.105 inch (0.27 cm.), and the panel will weigh about 5.4 ounces per square foot (1.65 kilograms per square meter). This panel is about the weight of the lightest commercially available "usual" panel, but it is considerably thicker. It also is at the upper limit of prism cell sizes which give an impression similar to that given by the usual panel. A prism cell side of about 0.017 inch (0.04 cm.) is at the lower limit of such sizes. Total thicknesses may range from about 0.07 inch (0.18 cm.) up. Cell sides of about 0.018 to about 0.020 inch (0.046-0.051 cm.) and panel thicknesses of about 0.10 inch (0.25 cm.) or less are preferred because they closely mimic the usual panel when viewed casually at a distance, and they represent an important savings of material as compared with the lightest of the usual panels.
The optical properties of the preferred embodiment of the panel of the present invention have also been found to be as good as, or somewhat better than, the optical properties of the light-weight usual panel. Of course, neither of the panels performs as well optically as the usual panel having an overall thickness of 0.12 inch (0.30 cm.).